1. Field of the Invention
The present invention is generally directed to the field of flow molding, and is more specifically directed to a unique flow molding process for forming a hardened skin on a surface of a molded article.
2. Description of Related Art
Various flow molding apparatuses are known in the art that employ dielectric heating to mold a plastic part from a formable plastic material. In all of these apparatuses, the plastic material is placed between two electrodes such that the material effectively becomes the dielectric of a capacitor. An alternating electric field generated between the electrodes causes polar molecules in the plastic material to be attracted and repelled by the rapidly changing polarity of the electric field. The friction resulting from this molecular movement causes the plastic material to heat throughout its entire mass to thereby form the molded article.
One flow molding apparatus known in the art for making plastic parts comprises a top electrode and a bottom electrode with top and bottom molds disposed therebetween. The top and bottom molds define a molding cavity in which a plastic material may be placed. Preferably, the current field lines are perpendicular to the plastic material at all points along its surface to thereby provide a uniform temperature throughout the material. In addition, the top and bottom electrodes substantially match the configuration of the plastic part that is being fabricated such that the distance between the electrodes is constant in order to provide uniform heating of the plastic material. In operation, an alternating electric field is applied across the molding cavity to thereby form the plastic part. An example of this type of a flow molding apparatus is disclosed in U.S. Pat. No. 4,268,238.
Another flow molding apparatus known in the art for making plastic parts comprises a top electrode and a bottom electrode with a mold disposed therebetween. The mold has a non-uniform thickness so as to allow the molding of a non-uniform plastic part from a plastic material placed between the mold and the top electrode. In order to provide uniform heating throughout the plastic material, a constant capacitance is maintained throughout all of the different thickness sections of the plastic part. This may be accomplished by equalizing the relative dielectric constants between the plastic material and the mold, preferably by altering the relative dielectric constant of the mold via the use of additives. Alternatively, the capacitance may be equalized by modifying the spacing between the top and bottom electrodes in the different thickness sections of the plastic part. An example of this type of a flow molding apparatus is disclosed in U.S. Pat. No. 4,441,876.
Another flow molding apparatus known in the art for making foamed plastic parts comprises a top electrode and a bottom electrode with a mold disposed therebetween. A plastic foam material may be placed in a cavity of the mold and then compressed during the heating cycle. After the heat is terminated, the compressed plastic foam material is permitted to expand as it cools so as to conform to the shape of the mold and thereby form the foamed plastic part. An example of this type of a flow molding apparatus is disclosed in U.S. Pat. No. 4,524,037.
Yet another flow molding apparatus known in the art for making foamed plastic parts comprises a top electrode and a bottom electrode with a two-piece mold disposed therebetween. The mold supports a diaphragm such that a plastic foam material may be placed between the diaphragm and the bottom mold. A fluid is injected into the mold above the diaphragm so as to initially deflect the diaphragm and thus expel substantially all of the air from the mold. The fluid is then extracted from the mold during the heating cycle, which causes a vacuum in the mold to thereby assist in the expansion of the plastic foam material. An example of this type of an apparatus is disclosed in U.S. Pat. No. 4,851,167.
The flow molding apparatuses and related methods described above are suitable for the manufacture of many different types of plastic parts, including foamed plastic parts. Many of the foamed plastic parts made in accordance with these methods, however, are not sufficiently durable to withstand the abrasion that occurs during normal use of the parts, are not easily washable, cannot be texturized as desired, and/or do not include a non-skid surface as required for particular applications. Thus, there is a need in the art for a molding process that overcomes one or more of the problems associated with the methods for forming the foamed plastic parts described above.